Clarify `FPDF.add_font()`'s `style` parameter accepted values

[jhenriquelc]

Changes:

In FPDF.add_font(), a line of documentation about the accepted values of the style parameter is being clarified to explicitly mention the empty string case and infer that "BI" is an accepted value.

Motives:

I suspect this is the cause for a misimplementation in typeshed's typing stubs, which I have opened a PR there to fix.

Sorry for this small PR, but I felt like it was appropriate. I'm looking forward to doing more (and bigger) docs clarifications if they're welcomed.

Thank you for working on this amazing project, maintainers!

Checklist:

• [x] This PR is ready to be merged

• [ ] The GitHub pipeline is OK (green), meaning that both pylint (static code analyzer) and black (code formatter) are happy with the changes of this PR.

• [N/A] A unit test is covering the code added / modified by this PR

• [N/A] In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder

• [x] A mention of the change is present in CHANGELOG.md

License consent:

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[Lucas-C]

Thank you for your contribution @jhenriquelc 👍 I'm going to merge this PR now.

@allcontributors please add @jhenriquelc for documentation

[allcontributors[bot]]

@Lucas-C

I've put up a pull request to add @jhenriquelc! :tada: